Disposition of fly ash, which is essentially fine solid non-combustible mineral residues (not to be equated with bottom ash, cinders, or slag) from coal-burning installations such as electric power generating stations is an increasingly difficult problem. Recent and increasingly strict environmental restrictions pertaining to the handling and final placement of fly ash, and the currently available technology (which was developed without prime importance being attributed to environmental considerations or changing ash characteristics) have combined to make the wasting of fly ash a source of rapidly escalating costs and environmental concerns common to nearly all coal burning plants.
The disposition of liquid wastes which are generated as by-products of various industrial processes also poses a difficult problem. These wastes, categorized as either hazardous or non-hazardous by various regulatory agencies, pose an unusual disposal problem in their natural state, and an increasing amount of emphasis is being placed on solidifying these liquid wastes in the disposal process. The technology by which this is accomplished is developing; however, all processes developed to date significantly increase the cost of disposal of these materials.
One of the greatest sources of fly ash, and a source which invariably impinges heavily on populated areas, are electric power generating stations. Accordingly, the invention of this application will be described in terms of the handling (in a transporting sense) and the treatment of fly ash which is generated at a power station. It should be understood however that the inventive concepts described herein are applicable to any source of fly ash generation.
Water for the treatment of fly ash can be obtained from ground or surface water sources dependent on availbility.
Liquid wastes employed in the disclosed process are produced as a by-product of industrial processes, which liquid waste materials must themselves be disposed of in accordance with environmental requirements.
The liquid waste used in the embodiments disclosed in this application was from the equipment manufacturing industry. This disclosure will pertain to transporting and treatment of liquid wastes, or water from a selected source, along with fly ash to form an acceptable landfill mixture. However the inventive concepts described herein are applicable to any source of liquid waste and water.
The methods of wasting fly ash which traditionally have been utilized in connection with electric power plant operations are the "sluicing" method and the "dampening" method.
The sluicing method, of wasting fly ash requires a liquid content of greater than 90%. Liquid content is defined as the ratio of the weight of the liquid divided by the weight of the dry fly ash expressed as a percent. In the water sluicing process a small amount of fly ash is mixed with a large amount of liquid and pumped into a holding pond or cell. As a consequence this process has several disadvantages when considered in relation to current commercial and environmental conditions. On many occasions such quantities of liquid are not readily available, or, are only available at a very high cost, such as in arid regions. A liquid, such as water, is becoming an increasingly valuable natural resource which must be protected. Further, expensive site preparation such as special cell liners and embankments for liquid decanting and liquid recycling facilities must be employed.
In the dampening method of wasting fly ash, just sufficient liquid is added to the dry fly ash to calm the dust. Although the liquid content will vary with the type of fly ash (which of course varies with the types of coal burned in the boiler, and its condition--i.e., crushed or pulverized) a liquid content of about 5% is the minimum lower limit while about 25% is the maximum upper limit of liquid in this process.
Current dampening systems commonly mix a continuous flow of fly ash with a continuous flow of liquid. Although the liquid addition rate can be controlled, the fly ash will not flow on a uniform basis and, as a result, the liquid content of the dampened fly ash will vary from practically nothing to 50% or more. This variability produces handling problems in that at the lower end of the liquid range fugitive dust is generated; on the upper end, the ash is turned into a paste or sludge which will not empty from a conventional dump truck. Further, the variability of liquid contents typically causes difficulty with landfill compaction. This impacts negatively on the environmental acceptability of the disposal operation.
The existing dampening systems are becoming less and less suitable for today's needs since fugitive dust is always a problem and handling problems are invaribly encountered particularly with changing fly ash chemical characteristics.
Liquid control can be critical for certain types of fly ash. Fly ash materials are classified by ASTM as type "C" or type "F". A type "C" fly ash has a relatively high calcium content and reduced iron content (as compared to a type "F" fly ash) and in many instances behaves as a natural cement when mixed with certain quantities of liquid.
Attempts have been made in the past to utilize dampending systems with Class C fly ashes. However, because of the chemically reactive nature of Class C fly ash and the lack of liquid content control in mixing, many operational and environmental problems have been encountered. In this connection it should be noted that, in the past, the liquid content control needed to adequately process Class C fly ashes is much more important than with Class F fly ashes. Also, environmental rules require greater control for disposing of all types of fly ashes to control fugitive dusts, and produce a sound structural landfill. In the existing systems of dampening fly ash, the flow of fly ash from dry storage is typically non-uniform, not continuous, because of inherent fly ash handling problems, while the flow of liquid is typically uniform. For example, the fly ash and liquid may be mixed in an inclined rotary drum mixer or vertical conical mixer, each of which turns continuously while the materials are being combined. The mixing time within this type of mixer is typically under 10 seconds, which results in a non-uniform end product due to inadequate mixing. Another type of mixing involves filling a turbine mixer from a rotary vane feeder, which in turn is fed from a fly ash storage silo.
The rotary vane feeder is allowed to turn for a given length of time, then shut off and a predetermined amount of water is added to the fly ash and the combination mixed. However, since fly ash never flows from a silo uniformly over time, the amount of ash in the mixer will be non-uniform, resulting in variable liquid contents; indeed, the liquid content may vary from nearly 0% to 50% or more. When using this system on Class C ashes, many problems arise, some environmental, some operational. When too little liquid (typically under 10% depending on the ash) is added to the fly ash, the mixture will be too dry and dusting problems will be encountered in the loading, transport and landfilling operation of the ash. When too much liquid is added (typically above 25% liquid content) the mixture becomes either very sticky and hardens quickly, thereby plugging mixing equipment or fails to discharge from a conventional dump truck, or the mixture will be too soupy and leak from the dump truck during the trip to the landfill.
While this uncontrolled system generally produces a poor landfill material, equipment maintenance problems are an even greater concern.
As a result, typically, this essentially uncontrolled liquid content mixing has been unacceptable with most Class C fly ashes.
There is typically a liquid content range (10%-15% for Class F, 15% to 25% for Class C, depending on the particular fly ash and liquid used for mixing) where a uniformly dampened fly ash will act like a conditioned fly ash. As a conditioned mixture, dust is not a problem because all the dust is combined with water and wetted in contrast to the dampening process. The conditioned mixture will not be so sticky as to plug the mixing equipment in contrast to the dampening process. It can also be placed into a dump truck and hauled many miles to a landfill and not stick or harden in the bed of the truck at the landfill.
In effect, it is essential to control both the fly ash and liquid going into a mixture to assure the resulting mixture will have a liquid content that will not cause dusting problems from too little water, or sticking, plugging, or leaking problems from too much water.
The wasting of liquid wastes historically has been accomplished using very crude methods. Typically these liquids are transported to a disposal site in sealed tank trucks. These trucks are routed in conventional landfill operations to the point of general refuse disposal. The liquid wastes are dumped into the refuse, such as garbage, and mixed with a compacting bulldozer. The theory is that the refuse will soak up the liquid wastes, thereby reducing the impact on landfill conditions. The amount of liquid waste which can be mixed with garbage is controlled by regulatory agencies. Nevertheless, landfills have commonly become saturated with liquid waste, thereby generating environmental hazards and concerns.
Accordingly it is the primary object of this invention to provide a process and system for wasting fly ash and liquid wastes and/or water, which is economical, environmentally acceptable, and minimizes the major disadvantages of all currently known systems, including the sluicing, slurry and dampening processes described above.
A specific object of the invention is to provide a method of and system for wasting fly ash whereby fly ash is transported from a source, such as a power station's storage silo, to the disposal site in a manner which eliminates virtually all fugitive dust.
A further object is to provide a method of wasting fly ash in which the final mixture will have a liquid content of about 5% to 25%, and in which the liquid content and the density of the mass will be uniform, controllable and with maximum density.
Another object of the invention is to provide a process of wasting fly ash and a system therefore which results in no bleed water being generated at any point in the process.
Yet another object is to provide a fly ash wasting system which lends itself to control of sulfur efflorescence which is particularly advantageous in connection with handling fly ash.
Yet another object is to provide a process and system of wasting fly ash in which the processed fly ash can be transported many miles, placed, and compacted into a hard and stable mass which will support significant structural loads of up to several tons per square foot.
Yet another object is to provide a fly ash wasting system which requires only conventional, mechanical earth-moving means to emplace a mixed product having a damp-earth consistency, which product may therefore immediately be mechanically spread or compacted.
Yet another object is to provide a fly ash wasting system in which the final, hardened product meets all current environmental requirements, and is resistent to percolation from rainfall thereby reducing the pollution potential in the disposal area.
Another object is to provide a controlled system of mixing uniformly the fly ash and liquid so that the liquid content of the mixture is controlled to within 1% of the designed optimum.
Another object is to provide an environmentally sound method of disposing of liquid wastes by mixing said liquid wastes with fly ash to produce an end product and landfill mixture which is structurally stable, environmentally sound and yet economical to produce.
Another object is to recycle surface water runoff from within the landfill site in the processing system of fly ash disposal.
Yet another object is to provide a system which can be operated at the fly ash source, liquid waste source, landfill, or other location depending on economical or environmental considerations.
Yet another object is to provide a system which can change rapidly and accurately the liquid content of the conditioned mixture to accommodate chemical/physical changes in the fly ash or liquids.